“I have a decision to run again for the Senate,” Mr. Smith told WMUR. “This is nothing personally against Jeanne Shaheen, who I like as a person, but I couldn’t disagree more with her politics.”

Former state Sen, Jim Rubens and local activists Karen Testerman have also announced they are seeking the GOP Senate nomination.

Former Massachusetts Sen. Scott Brown also has been thought to be pondering a run, but the Republican has yet to enter the race.

Mr. Smith served in the U.S. Senate from 1990 to 2003. He lost his seat in a primary with John E. Sununu, who went on to win the general election and serve for a term before being defeated by Ms. Shaheen.

Before that, Mr. Smith served in the U.S. House from 1985 to 1990.

Mr. Smith has had an odd relationship with the GOP, including making a run for the party’s presidential nomination in 2000, then dropping out of the party to run for president as a conservative independent, then rejoining the GOP after that bid failed.

Republicans need to pick up a net of six seats to win back the Senate.